decay of high level waste

Factual Questions
1

I understand that high level nuclear waste is dangerous for a long time. Fuel assemblies contain a mixture of highly unstable fission daughters, neutron emitters, U-238 and transuranic elements. I also know that the half-lives of these products vary widely, from a few years/decades (fission daughters) to tens of thousands of years (some of the transuranics) to millions of years to a half-billion years (U238).

So for the first few decades the lighter fission products account for the majority of the radiation produced. At some point plutonium and the other transuranics become dominant for, let’s say, 100 kyears.

My questions are:

(1) At what point in this process does the radioactivity fall to the level of the original uranium ore? Decades? Thousands of years? Millions of years?

(2) The total volume of the waste is small, but if you reprocessed it to recycle fissile U-233, U-235 and Pu-239, could you also (to some extent) separate out short-lived elements from long-lived elements and cut back the total volume of material requiring extreme long-term storage?

2

Bump from page 2, because I’m curious, too. There’s never a nuclear physics geek around when you need one.

3

I don’t have a simple answer for you, because the compositions of the fuel elements will depend on the process used and their original composititon. In general, the highly radioactive transuranics tend to have short half-lives, and thus a much shorter but more intense period of radioactivity. They also tend to be alpha emitters which are really only dangerous if inhaled. The exceptions are those like [sup]240Pu, [sup]241[/sup]Pu, and [sup]252[/sup]Cf, which have short half-lives but are beta emitters and have high rates of spontaneous fission, which released damaging neutron radiation. Natural uranium ore is roughly 99.3% [sup]238[/sup]U with about 0.7% [sup]235[/sup]U, so it would take billions of years to get back to that level, but the most dangerous radioactives (even with the breeding that goes on due to spontaneous fission) are gone in a few thousands of years.

If you reprocessed the waste to seperate out fuel-grade fissiles from the “neutron poison” higher actinides, then you could just use these elements, and in fact, you could breed specifically for them, seemingly paraxodically making energy out of nothing. (You’re not–the energy is already there in the nuclear bonds–but you end up with more fuel than you started with, which is pretty amazing.) You could also artifically fission the unstable elements via induced fission, i.e. shining a neutron source on them, to reduce them ot less hazardous products in a short period of time. The problem with both of these concepts is that it requires handling dangerously radioactive, toxic, and often caustic products which have the potential to do significant harm if accidentially released into the environment or should a foul up occuring in handling them. So, either you deal with the problem and take the risk, or you dig a really deep hole, stuff it all down there, and don’t talk about the problem. Given the political influences on nuclear power and the inherent nature of politicians to shy away from difficult, complex, and contriversial topics, we can leave it as a simple exercise to the reader which option is most likely selected.

Stranger

4

I’m not really a nuclear physicist, but since it’s the weekend I’ll try to answer.

First, the half-life and radioactivity of a given isotope are (obviously) inversely related. An element with a short half-life will be very active until, after a few half-lives have elapsed, it has essentially all decayed away. You’re trying to compare to natural uranium ore, which is predominantly [sup]238[/sup]U with a half-life of 4 billion years; so not until most of the elements with shorter half-lives have decayed away will the sample’s activity really approach this level. This may take hundreds of millions of years, if your sample has some long-lived isotopes.

Of course after such a long time the sample is not very radioactive any more; setting such a threshold probably doesn’t make much sense if you’re just trying for some measure of safety. (Also remember that the safety of a radioisotope, and the amount of shielding required against it, depends not only on its half-life but on its decay mode (alpha, beta, or gamma).)

This is somewhat complicated by the fact that other isotopes may decay into these short-lived products, causing a rapid cascade of several decays until reaching a relativly stable product. But the lifetime of the radioactive sample will be dominated by the lifetime of a few long-lived unstable isotopes, and in the long view all of the short-lived stuff can be ignored.

You can follow the likely decay paths of a given isotope (if you happen to know the composition of your original sample) on a table of nuclides like this one. The colors indicate lifetimes; if you’re interested in Myr timescales, for example, then you can safely ignore anything that’s not dark blue or black. For example, [sup]243[/sup]Am decays via alpha to [sup]239[/sup]Np, with a half-life of ~7Kyr (alpha decays move two rows down and two columns left). [sup]239[/sup]Np has a rapid beta decay (half-life ~2day) to [sup]239[/sup]Pu (beta- decays move one row up and one column left). Then [sup]239[/sup]Pu, alpha 24Kyr to [sup]235[/sup]U, alpha 700Myr to [sup]231[/sup]Th, beta- 1day to [sup]231[/sup]Pa, alpha 30Kyr to [sup]227[/sup]Ac, beta- 20yr to [sup]227[/sup]Th, alpha 20day to [sup]223[/sup]Ra, alpha 12day to [sup]219[/sup]Rn, alpha 4s to [sup]215[/sup]Po, alpha 2ms to [sup]211[/sup]Pb, beta- 40min to [sup]211[/sup]Bi, alpha 2min to [sup]207[/sup]Tl, beta- 5min to [sup]207[/sup]Pb, STABLE.

This looks somewhat complicated, but notice that from the long view there are really only four isotopes to be seen: the original [sup]243[/sup]Am (7Kyr), [sup]239[/sup]Pu (24Kyr), and [sup]235[/sup]U (700Myr), and [sup]231[/sup]Pa (30Kyr) (and the final Pb). The stable isotopes are generally clustered along a “valley” in this chart; for heavy elements the valley runs generally east-northeast, as you can see. Elements tend to decay northwest or southeast toward the valley (via beta decay/electron capture) if they are outside of it, and otherwise via alpha decay southwestward to a stable isotope.

As for isotope separation, the problems with processing radioactive material are mostly practical. The materials used for handling will probably end up somewhat radioactive (by contamination and by alpha capture), so you end up with a lot of low-level waste. This low-level waste is probably not actually very dangerous, but at least with current regulations it must be handled fairly carefully anyway. There is also the possibility of a handling accident to consider.

On preview, I see that Stranger covered most of this better than I did. At least I’ve got a link.